Batteries are often used to power portable electronic devices. After a period of use, the batteries in a portable device will become depleted and affect the performance of the portable device. When the energy level of the batteries falls below a certain threshold, the portable device will cease to operate. At this point, the batteries must be recharged or replaced. Accordingly, in many applications it would be beneficial to monitor the remaining charge in the batteries and alert the user of a portable device before the batteries are depleted.
One environment where monitoring the remaining charge in batteries is important is in portable external cardiac defibrillators. Portable cardiac defibrillators generate and apply a high energy defibrillation pulse to the chest of a patient to cause the patient's heart to stop fibrillating and return to a normal rhythm. Sometimes the application of a single defibrillation pulse fails to restore the patient's heart to a normal rhythm. In such an event, it may be necessary to apply additional defibrillation pulses. Portable external defibrillators generally use a battery pack containing a number of cells to power the defibrillator. The battery pack allows an energy storage capacitor to be charged in order to generate a defibrillation pulse. If the battery pack becomes depleted, the patient cannot be treated.
In order to assure that an external cardiac defibrillator is always ready for use, it is therefore advantageous to monitor the remaining charge in the battery pack. Battery monitoring provides an indication of when the battery pack is nearly depleted and needs to be replaced or recharged.
Various efforts in the prior art have been devoted to monitoring the remaining energy in a battery pack. One common approach entails measuring an output voltage from the battery pack while the battery pack is connected to an electrical load. As the battery pack is discharged, the voltage will typically drop. Variations of this approach involve measuring other battery parameters, such as the impedance of the batteries in the battery pack, to detect changes that indicate that the battery pack is nearing the end of its useful life.
One disadvantage of monitoring the output voltage from the battery pack to detect a drop in voltage is that the method is unable to accurately monitor the energy level of typical nonrechargeable batteries. FIG. 1 depicts the output voltage v from a single nonrechargeable lithium battery plotted with respect to time. The output voltage from the lithium battery remains relatively constant until near the end of its life t when the output voltage drops precipitously. Because of the steep drop in voltage, it is nearly impossible to give sufficient advance warning when the battery pack is nearly depleted. By the time the battery voltage begins to drop, insufficient energy often remains in the battery pack to power the electronic device for any appreciable period. This is especially true for devices which have a high energy utilization rate such as defibrillators.
Another approach for determining the remaining charge in a battery pack is disclosed in U.S. Pat. No. 5,483,165 to Cameron et al. Cameron et al. disclose a defibrillator having a main battery consisting of a number of identical battery cells connected in series. A sense cell is connected in series to the main battery. The sense cell is identical in type and manufacture to each of the main battery cells. A current flows through the main battery and the sense cell to a load. Because all the cells are identical, the sense cell and battery cells in the main battery should in normal circumstances be depleted at the same rate. However, Cameron et al. disclose a dedicated circuit for drawing additional, incremental current from the sense cell. The value of the incremental current equals the value of the current delivered to the load scaled by an arbitrary constant. The dedicated circuit drawing the incremental current includes a variable resistance connected between the sense cell and ground potential. The variable resistance is used to vary the value of the incremental current drawn from the sense cell.
The sense cell in Cameron et al. is used to estimate the remaining charge in the main battery. Since the sense cell is discharged at a quicker rate than the main battery, the voltage drop characteristic of battery cells at the end of their useful life will first occur on the sense cell. When the voltage drop on the sense cell is detected by a controller, a signal or other warning is generated to indicate that the main battery is also nearly depleted.
While the Cameron et al. approach offers some advantages over other prior art techniques for monitoring batteries, it also presents disadvantages. The need to draw an additional current from a sense cell adds undue complexity and, it will be appreciated, problems attendant with that complexity. For example, the Cameron et al. approach requires adding a dedicated voltage monitor and a variable resistance circuit to draw the incremental current from the sense cell. The additional components and battery contacts required to draw the incremental current increase the likelihood that a component in the battery monitoring system could fail and render the battery monitoring technique ineffective.
Apart from the increased likelihood of failure, the need to provide a voltage monitor and variable resistance poses other problems. The addition of the voltage monitor and the variable resistance increases the cost of a device incorporating the battery monitoring system. The voltage monitor and the variable resistance also add weight and size to the system.
The use of additional circuitry to draw an incremental current from the sense cell also limits the interchangeability of the type of battery cell incorporated in a battery pack. For example, if lithium battery cells are used in the battery packs, a certain incremental current must be used that corresponds to the anticipated voltage curve of the lithium battery cell. If there is an improvement in battery technology or if a user desires to use a different type of battery cell, such as a rechargeable nickel cadmium battery cell, a different amount of incremental current should be drawn from the new cell to correspond to the anticipated voltage curve of the new cell. An operator of the portable electronic device must therefore somehow adjust the amount of incremental current drawn depending upon the type of battery pack that is to be used in the device. The need to perform an additional adjustment limits the ease with which the user may select a type of battery suitable for a particular application.
In addition to monitoring the remaining charge in the battery pack, it would also be advantageous to monitor the health of the individual battery cells within the battery pack in order to detect a possible cell failure. Individual cell failures decrease the energy storage capacity of the battery pack and can also detrimentally affect the performance of the battery pack during discharge of the battery pack.
As can be seen from the discussion above, there exists a need for improved methods of monitoring the condition of battery packs. The present invention is directed toward addressing this need.